On Jun 22, 2007, at 3:11 PM, Brandon S. Allbery KF8NH wrote:
(1) any way to flag a pattern match as "I know this is okay", don't
warn about it" without shutting off pattern match warnings completely?
GHC doesn't issue warnings about patterns on the left of =
For example, the following code compiles just fine with ghc -Wall -
Werror, but the use of "Just m" generates a run-time exception:
module Main where
a :: [(Int,Int)]
a = [(2*n,n) | n <- [1..100]]
m :: Int
Just m = lookup 3 a
main :: IO ()
main = putStrLn $ show m
I'd take this as a ghc feature, not a bug. When I use this construct
in practice, I have a proof in mind that the pattern match cannot
fail for my data, but I can't express the proof in Haskell's type
system. I'm ok with skipping writing that proof.
The difference here is programmer intent. While a missing pattern
case can often be an oversight, there's no way to put both cases here
to the left of =, so the programmer clearly intends this code as
written.
(An example of a language with a Turing complete type system is Qi:
http://www.lambdassociates.org/
As pointed out elsewhere in this thread, it is unreasonable/
undecidable to expect a type system to work out arbitrarily difficult
issues for you automatically. Some work is required, programming in
the type system. They extend this point of view.)
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